1. Field of the Invention
The present invention relates to a radio transmitter used in the field of security and the like.
2. Description of the Related Art
For cellular phones, personal computers, and the like, there has been a demand for an automatic password locking/unlocking function or an automatic power on/off function. For automobiles, there has been a demand for an automatic door locking/unlocking function. To meet these demands, various methods have recently been proposed. In a proposed technique, a user carries a radio transmitter with an assigned unique ID. When the user leaves the cellular phone or personal computer, the password is locked. When the user approaches the cellular phone or personal computer, the password is unlocked.
The radio transmitter of this kind is preferably small and thin in connection with portability. In particular, to offer high portability, the radio transmitter is preferably shaped like a card with a minimized thickness which can be housed in a wallet or an employee ID card holder always carried by the user. For example, Japanese Patent Laid-Open No. 2004-363929 describes a keyless entry transmitter as a radio transmitter. The radio transmitter of this kind is carried by the user in a breast pocket, a back pocket, or the like depending on the user's usage. Thus, the radio transmitter preferably offers nondirectionality.
In the conventional radio transmitter, the antenna is designed to offer nondirectionality. However, the conventional antenna design assumes that the radio transmitter is not carried by the human body. Thus, when the user carries a radio transmitter having a radio module, a battery, and the like accommodated in an external case made of resin (for example, an ABS material), electric waves radiated toward the human body may be absorbed by the human body. This may reduce the antenna gain toward the human body. Such a phenomenon may reduce the intensity of the electric wave during radio communication, thus reducing the radio communicable range. Furthermore, the reduction in the antenna gain toward the human body may change the antenna directionality, thus preventing the nondirectionality from being maintained.
Thus, an object of the present invention is to provide a radio transmitter that enables nondirectionality to be offered without reducing the antenna gain while the radio transmitter is being carried by the human body.